Are films still appealing in the holodeck

Almost a full holodeck

Since “Star Trek” we have been dreaming of the ultimate display that can project audiovisual and haptic content into the room. Now engineers in the UK have created a 3-D display that can do just that. What is it good for?

It's actually about something very futuristic. About a device that sounds like a spaceport and an agent film. Ryuji Hirayama seems very down to earth. Anyone who asks the young engineer something first hears a torrent of words and units, and mumbling in a low voice, he calculates something. And then comes an answer that is so technical that all the images of a glittering world suitable for science fiction that one has in mind when thinking of a holodeck collapse. It's a shame actually. But maybe someone has to think and work that way who uses sound and styrofoam to create three-dimensional moving images in the middle of the room. Pictures that you can touch and that also still sound.

Light, air and a lot of electronics

Hirayama describes the prototype of his invention together with his colleagues from the University of Sussex in Brighton (Great Britain) and the Tokyo University of Science this week in the journal "Nature".1 The scientists make use of the principles of acoustophoresis; that is, they use sound waves to keep a small styrofoam ball in suspension and to move it. They also illuminate these with a color-changing LED. If the styrofoam ball is moved fast enough, pictures can be "written in the air" in this way.

Draw a picture in the air

The principle is as simple as its technical implementation in detail. For the researchers from Brighton, the display - i.e. the space that can be used for play - is a cuboid around 23 by 17 by 17 centimeters. Small loudspeakers (technically correct: transducers) sit close to each other on the floor and ceiling, which generate a field of sound waves that can be transformed in almost any way by cleverly controlling the 512 transducers.

The sound waves are able to keep sufficiently light particles - such as the small styrofoam ball that Hirayama and his colleagues used - in suspension. Their position is determined by the sound waves, because small particles always move to areas where the fluctuations in air pressure are as small as possible.

Standing wave

The publication is accompanied by impressive pictures and videos: Numbers and symbols flash, and the globe with its continents in miniature. A colorful butterfly flutters past. However, the researchers have already tricked them a bit: It currently takes 20 seconds for the 1 mm large styrofoam ball to move to all the positions that are necessary for displaying the globe. So large and complex images cannot be generated “live”, but only with the help of long-term exposure on a camera. And because the control of the styrofoam ball using sound waves is not perfect, the researchers also built in a tracking system that continuously determines and corrects the position of the ball in the display. Without this, the small styrofoam ball would fall out of its sound wave bed in no time.

In addition to moving images, the display can also play sounds by modulating the ultrasonic waves that are required to control the styrofoam ball (40 kHz) with the appropriate, audible frequencies. Even the sense of touch can be addressed with the “multimodal acoustic trap display” (MATD) - as the engineers from Brighton call their invention. This is because the sound waves generate a noticeable vibration that can also be modified via the loudspeakers. All in all, even if you take into account the limitations of the prototype, it is quite close to the holodeck, this “ultimate display” that can represent any environment audiovisually and haptically.

Just a nice toy?

Dimos Poulikakos, who heads the Energy Science Center at ETH Zurich and who has himself driven various developments in the field of acoustophoresis, thinks the work is original and well done. The basics of the technologies used have long been known for themselves, but are cleverly combined into a whole. However, it is difficult for him to imagine a specific application or even a suitable product into which the invention could result. Electrically controlling all 512 transducers so that the acoustic field exerts the necessary forces on the particle is complicated, says Poulikakos. In addition, there are already holographic display technologies that create three-dimensional visual impressions with relatively little effort. “Can the new technology, even if it is further developed, compete with the existing ones?” Asks the researcher.

Another point of criticism concerns the resolution that is feasible with the MATD. It is essentially determined by the size of the illuminated styrofoam ball. "Here we have technologies that are far better and already work," emphasizes Poulikakos. In addition, there is no evidence that the new technology could outperform existing holographic 3-D displays in terms of resolution and image size.

The question remains whether the Hirayama method could also be used to play in much larger rooms. Hirayama hesitates - as always - before giving an answer: You have to have much more powerful transducers, he says, and use several particles at the same time. We are working on both. Poulikakos is less optimistic: With a larger volume and use outside the controlled laboratory environment, there are many outside influences. Somebody just needs to cough, and the field of sound waves is disturbed. Nevertheless, the originality of the idea is remarkable.

1Nature 575, p. 320-323 (2019).